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Community and Q&A

New home HRV with massive shower/hot tub and range hood

kleach | Posted in Energy Efficiency and Durability on

I am building a new 3500 sq/foot home in New England. Well the 3500 does not count the finished basement of another 1000 nor does it over the “attic” space. The house is being framed now and we plan on spray forming the rafters. It has zip system on the exterior sheathing & taped. So the house will be tight.
Here is my issue… the house will have a “gourmet” kitchen which translates to a 48″ range with a crazy 150,000 btu cook top. If you use the standard rule of thumb if 100cfm for 1kbtu.. that is a 1500 cfm externally mounted blower for the range hood…. yea..
ALSO, there is an insane master bath, Hot tub and a dual master shower with over 16 gpm possible (but unlikely they would use that all at the same time.) The steam output could make it rain in there… the bathroom itself is large but the hallway to it from the master bed includes a door on either side to walk in closets and no one wants moldy clothes. The rapid humidity removal of all that steam is important…
The HVAC system will be geo thermal and there is radiant heating for the basement slab.
They have THREE bathroom exhaust vents lined up for the master bath one for the “shower” or as i call it the waterfall… one for above the tub and one for the head. They are 150cfm, 110cfm and 80cfm respectively. = 340 MORE cfm for a whopping total of 1840 cfm!
This does not count the three other 150 cfm exhaust fans for the remaining showers in the house. (there are 4 in total) and 1 1/2 bath.
SO the “tight” house isn’t so tight anymore.. granted not all of this would be on at the same time in all likelihood.. but still I need to have a HRV system to keep the air “fresh” and the house balanced.

The good news.. NOTHING Is ordered yet! We have a chance!
As I said they are framing and will be adding roofing soon… which means I want the vertical, roof penetrating vents to be installed now before they shingle.. The propane fireplace comes to mind.. but what of this issue with all these exhaust outlets?

I am sure this will start a great debate… but in short time I have to decide what to order / build…
I would like to opinion of the talented people here.. as I have read many posts and I am curious what the consensus will be.

Thanks in advance.

On a side note.. I would happy to build this based on the suggested solutions and report back once built and a year after to show the issues if they are any…


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  1. iLikeDirt | | #1

    There is some irony to discussing such an enormous, lavish project in a green building forum. ;-) The environmental impact from this house is going to be gigantic any way you slice it. Any opportunity to meaningfully conserve the planet's resources for future generations was already lost in the planning stage. I say just build code min and pocket more profit; little you do at this point will make much of a difference for this house's lifetime energy and resource consumption. And since it's made out of wood, it will be easy to demolish once ridiculous houses like this fall out of fashion. Just make it work during it's sad, short lifespan--don't try to make it green.

    An HRV is not going to be capable of providing 1840 CFM of make-up air. That's not what it's for, anyway. Unless you build the building envelope entirely out of welded sheet metal, there will be enough air paths available for the make-up air to enter when all those exhaust fans are hypothetically running, especially with such a huge surface area for the building envelope. If this house will have a zillion corners, bump-outs, and turrets, then you're not going to be able to air seal it well without herculean effort anyway, so why bother? The make-up air won't be a problem.

    However, such a huge exhaust airflow will probably depressurize any atmospherically vented combustion appliances, like the mentioned propane fireplace. Unless it's some kind of sealed model that draws air from outside and exhausts there too, totally disconnected from the indoor air, then that huge air movement in the house is going to make it backdraft. Plan for all sealed combustion appliances or else electric.

    If there is going to be any stone veneer, be sure to read

  2. STEPHEN SHEEHY | | #2

    Unless runing a restaurant, you're not likely to be using the entire 150,000 btus at any one time, but you'll still need a big hood fan and a dedicated source of make-up air for it.

    Absolutely install a sealed combustion fireplace that can vent through the wall, not the roof.

    Your hrv should be designed with a boost function to handle the other three showers, so you might skip the vent fans for them.

    You say "hot tub." Do you really mean a hot tub always full of water, with an insulated cover? Or a Jacuzzi that needs to be filled each time. If the latter, i wouldn't bother with a dedicated exhaust fan, because no one will ever use it. If a real hot tub, they don't produce much water vapor unless you leave them uncovered, which in a conditioned space is a bad idea. Still, an hrv boost is a good idea.

    Is the master bath three distinct rooms? Otherwise, why three fans? My much more modest bath with dual shower heads gets by fine with just the hrv boost, no separate fan. The room is 6' x 15' or so. No tub though.

  3. LucyF | | #3

    Is it April 1st already?

  4. charlie_sullivan | | #4

    Nate says that this is such a disaster that it's not worth trying. I urge the opposite. If you put your efforts into reducing the energy use of an energy-hog, and successfully reduce it by 15%, that makes a much bigger difference than reducing the energy use of a energy sipper by 40%. It's much more fun to achieve excellence, but the nearly excellent houses aren't the problems--it's the terrible ones that need help.

  5. kleach | | #5

    From the energy side I did not mention that it is has a of 10k solar on the roof.. ..which makes it suck less.. the tub is not constantly filled so yes.. I agree the extra fans not worth it.. the question is the master shower can make tons of steam.. so do I just go with a booster or install a dedicated fan?
    For the range hood, I am looking into a compensating hood that is close to neutral... but would come in unheated.. not sure of my options.. obviously there is a working stove/cooktop there with heat.. .
    For the propane, currently it is setup with BLOWER I found out today... so more air .. from where I don't know.. I have to look at the plans.. which I may ignore..
    The propane fireplace at full throttle can make over 50,000 BTUs! and is direct vent..

    I know this house is huge.. but I am trying to do what I can to make it work... like Charlie said.... basically .. make it suck less and as safe as possible..

    Thanks again for the discussion ..


  6. LucyF | | #6

    Ken, I am sorry about my comment. I just don't understand a halfway commitment to energy savings. I feel guilty about leaving a light on to make it easier to return home at night.

    Good luck in reducing the environmental impact of this house. Maybe you will make it so nice that they actually appreciate your efforts at energy efficiency.

  7. kleach | | #7

    The house uses the zip system sheathing.. and is fairly air tight (thus far) without a any spray foam yet. The geo will run off of the soalr for the most part and will bank credits during summer for winter use. The spray foam should seal the remaining "leaks". I am removing the bathroom vents and using a boosted HRV system and the range hood will be zero loss of air not heat. The basement floor is radiant powered from geo by product. There is a well system for water and we are looking into finding a legal way to capture rainwater for lawn sprinklers.

    My point is why can't a larger home be mostly green?
    What else would you have me do? or do a different way?

    I don't expect this house to be zero impact.. no garden in the roof etc.. but there has to be a reasonable middle ground on function/price.

    Is there a better way to vent the range? aside from not having it...
    A lot of money is being spent up front to make the run rate less for this home... just trying to make sure we do it right.. whatever that means..


  8. dinnerbellmel | | #8

    I can't offer any specific solutions but just wanted to congratulate you. The house sounds great and I applaud your efforts in thinking green. It's great that people are starting to think in this fashion and actually working towards making their homes more efficient and reducing the carbon footprint from the get-go!

  9. STEPHEN SHEEHY | | #9

    ken- we really don't know much about your house and whether it is very energy efficient or not. We know it's a big house.
    I'm assuming this is your house, not that you are the GC for someone else. Some questions:
    Do you have an airtightness target? Is it in the contract?

    What sort of spray foam insulation are you planning, how much, what R-values are you aiming at in roof and walls.

    How are the walls and ceiling/roof framed?

    What about windows and doors? What type, SGHC,U-values?

    Why a geothermal heat pump? Do you have good heat loss calculations, such that you know the system is properly sized?

    Some decisions have already been made, but some folks here may have some ideas that can still make a difference in energy use and comfort.

  10. kleach | | #10

    I'm assuming this is your house, not that you are the GC for someone else. Some questions:
    I am both.. GC and current owner... what I say goes..

    Do you have an airtightness target? Is it in the contract?
    We’re shooting < 4%, what do you suggest as a target?

    What sort of spray foam insulation are you planning, how much, what R-values are you aiming at in roof and walls. How are the walls and ceiling/roof framed?
    We are open to this conversation. The original plan was open cell everywhere.. we were have 6” walls and 12” rafters (stick built). The current conversation was to save money.. we put 1” of closed cell on walls then batts… on roof.. same but with open cell on top? This way we get the water/air tight with closed and the insulation with closed or other option.. we looked at blown in but want to use the storage area not as cold storage. BUT the roof pitch is either 10 or 12 (12 hip, 10 main common rafters). This means there is a LOT of surface area, and we were looking for more cost effect ways to insulate at the same levels….
    What about windows and doors? What type, SGHC,U-values?
    Anderson 400 series Low-E4 Northern Climate Zone Looks like .28 U factor? Seems low
    Why a geothermal heat pump? Do you have good heat loss calculations, such that you know the system is properly sized?
    Yes a calc was done but honestly the numbers seem academic. The current spec is for a 6 ton unit with two 400’ wells.. and an H/ERV system to recover the air.. Again, this has not been ordered yet (soon) so we still have options.. there is radiant pipes in the slab in the basement…

  11. Expert Member
    Dana Dorsett | | #11

    I don't know how you do a heat load calc unless you know which windows you're using, the ultimate R-values and air tightness, etc. Most pro-forma heat load calculations use code minimum assumptions and GENEROUS air leakage assumptions, and that's the crummiest house that's legal to build. You can save a lot of money on mechancial systems and buy a lot of comfort by going better than code-min on the building envelope. You probably won't get it down to 2 tons, but you can probably get it down to 3 on a house that size. (I've recently seen sub-code 3000-3200' houses in New England with 3 tons of air-source mini-split heating them, so I think the 6 ton GSHP proposal assumed you sleep with the windows open. :-) )

    Seriously, design the house first, and in the process have an engineer calculate the the heating & cooling loads as the design gets tweaked- don't leave it up to the GSHP installer. With the hard numbers in hand THEN solicit bids on the geo, and be ready to push back on sizing.

    Got an approximate outside design temperature, and or a ZIP code?

    Regarding the 16gpm monster-shower, that's going to take a HELL of a lot of hot water heater to keep up, whether a large tank or something with huge BTU output. A 16 GPM even 100 gallon tank goes pretty fast, and would have a long recovery rate if it's being heated with the GSHP. A 16gpm flow is about 8000 lbs per hour, and at a 70F rise (35F water in, 105F at the shower heads) would require an 8000 x 70F= 560,000 BTU/hr of burner output just to keep up.

    Were you planning to gang three 199K propane tankless water heaters together, or install a couple of 100 gallon tanks or...???

    Think seriously about integrating drainwater heat exchangers down stream of the showers into the mix, which could cut your burner or tank-size requirements roughly in half without burning more fuel. A single 4" x 72" gravity film type heat exchanger would have a steady-state return of about 60% at 2.5 gpm, about 50-55% at 5 gpm, but you'd probably need to parallel up two of them to manage 16gpm without a large drop in pressure over the units. They not only provide the greater capacity, they also provide a huge efficiency factor.

    The version with the least pressure drop with flow is Renewability's PowerPipe series

    They're not cheap- direct from manufacturer is often higher than through distributors:

    There are others, but you may have to go with 4 paralleled together to support 16gpm flows with some of the more restrictive vendors- it gets to be a complicated assembly.

  12. STEPHEN SHEEHY | | #12

    Ken- I'm not familiar with expressing air leakage as a percentage. Typically, it is expressed as air changes per hour at 50 pascals of pressure. A blower door test will give you air leakage in cubic feet per minute at 50 pascals. Multiply that number by 60 to get cubic feet per hour and then calculate the total cubic feet of conditioned space. Divide the cfh by the volume in cubic feet and you'll get a number like 1.0ach50.
    A blower door test can be done at various stages, but should be done while you can still seal any leaks you find during the test, Obviously, do it after windows and doors are in and after you have installed whatever air barrier you end up using.

    I believe current best code requires 3.0ach50, but in a new house you should do much better than that. Pay very close attention to air sealing and you can hit 1.0ach50. Better numbers translate into better energy efficiency.

    Listen to Dana about heat load calculations. As you'll see from other posts around this site, HVAC installers routinely oversize systems. Regarding windows, U.28 is not great for a cold climate.

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